Various casting molds and casting methods are used for metal casting, and as a metal casting methods for rapidly casting an metal product, that is, a metal casting method for rapidly cooling molten metal, a method is used which rapidly injects molten metal into a mold cavity by using gravity or by drawing the molten metal and transfers heat to the mold, thereby casting a solid metal product from the liquid molten metal.
The casting method is mainly used to manufacture a specimen made of an amorphous alloy, but there is a problem in that it is necessary to rapidly cool molten metal during the manufacturing process, at a speed higher than a speed at which metal elements form crystal structures, even though the amorphous alloy has no crystal structure and thus has better physical properties such as rigidity than general metal.
As a method of manufacturing the amorphous alloy, as described above, a differential pressure type casting method is used which allows the molten metal to be introduced into a casting mold by gravity or draws the molten metal into a mold cavity by using negative pressure. According to this method, the molten metal is rapidly cooled while flowing through a narrow passageway and filling the narrow passageway, but there occurs frictional force between the molten metal and the passageway while the molten metal flows through the narrow passageway.
Because the method of introducing the molten metal into the mold cavity by using gravity has a problem in that the molten metal cannot quickly flow into the mold cavity due to the frictional force in the narrow passageway, a method is mainly used which casts an amorphous alloy specimen by rapidly drawing the molten metal into the mold cavity by using negative pressure made by suction and by solidifying the molten metal by transferring heat to a main mold body having high thermal conductivity before crystallization of metal, and a casting mold used for the method is mainly used.
As a typical example of a casting mold used for the differential pressure type casting, FIGS. 1A and 1B illustrate a casting mold used to cast a bar-shaped specimen made of an amorphous alloy.
A casting mold 100 includes a main mold body 110 having a mold cavity 111 having a circular cross section that matches with a bar shape of a specimen to be manufactured, and an upper mold 120 which is placed on an upper end surface of the main mold body and has a support portion 121 formed at an upper side of the upper mold 120 so that a metal to be melted is placed on the support portion 121.
The mold cavity 111 has a shape extended from the support portion 121 to a lower end surface of the main mold body 110, and a stopper 130, which prevents a leakage of molten metal, is disposed at a lower end of the mold cavity 111. The stopper 130 has four suction holes 137 that extend from an upper end surface 132 of the stopper 130 to a lower end surface of the stopper 130, and a non-illustrated vacuum suction source is connected to the suction hole 137.
The suction holes 137 are exposed at the upper end surface 132 of the stopper 130, but a portion of the upper end surface 132 of the stopper, where the suction holes 137 are exposed, is in contact with a surface of a stopper insertion groove 112 formed in the lower end surface of the main mold body 110 but is not in direct contact with the mold cavity 110.
Meanwhile, a heating source 2 for heating the metal 1 is disposed above the upper mold 120, and an arc electrode 3 is disposed in the vicinity of the metal and generates an electric arc, thereby melting the metal.
When the metal 1 is melted, the vacuum suction source is operated to perform suction through the suction holes 137. Upper ends of the suction holes 137 are in contact with the surface of the stopper insertion groove 112, but because the upper end surface 132 of the stopper has minute scratches or unevenness caused by machining and thus has high surface roughness, air is suctioned from the mold cavity 111 through the scratches or the unevenness, such that the molten metal 1 on the support portion 121 is drawn into the mold cavity 111 by negative pressure.
A bottom surface of the mold cavity 111 is closed by the upper end surface 132 of the stopper to the extent that the molten metal does not leak to the suction holes 137, such that the molten metal 1 fills the mold cavity 111 from the bottom surface of the mold cavity 111.
The main mold body 110 is made of a material such as copper having high thermal conductivity, and particularly, the main mold body 110 is configured such that a cooling fluid circulates inside or around the main mold body 110, and as a result, the molten metal, which is drawn into and thus fills the mold cavity 111, is rapidly cooled and solidified before crystallization thereof, and formed as amorphous metal.
This type of casting mold is used to cast a bar-shaped metal product, but the inventors of the present invention have made a casting mold illustrated in FIG. 3 in order to use this type of differential pressure type casting mold to manufacture an amorphous alloy in the form of a sheet.
A casting mold 200 illustrated in FIG. 3 is configured by changing a part of a drawing passageway 211 for a molten metal in the casting mold 100 illustrated in FIG. 1, which is vertically extended from the support portion 121 at the upper side to the upper end surface 132 of the stopper at the lower side, into a mold cavity 213 which has a small thickness and a large area so that a metal sheet may be formed, and by allowing the mold cavity 213 to have a shape that is narrowed toward an upper end portion and a lower end portion thereof.
However, several problems are found as a result of casting an amorphous alloy sheet by using the casting mold 200 configured as described above.
First, it is ascertained that casting defects occur at a severe level in the casted alloy specimen because the molten metal non-uniformly flows from an upper end to a lower end of the mold cavity 213. In addition, a discontinuous interface, which is formed due to a difference in metal flow between left and right sides with respect to a vertical path from the upper portion to the lower portion in the mold cavity 213, is observed between the left and right sides.
Therefore, it is ascertained that an appropriate amorphous metal sheet cannot be casted by using the configuration of the differential pressure type casting mold in the related art as it is.
As another method of manufacturing a metal sheet made of an amorphous alloy, there is an invention related to an apparatus and a method of manufacturing a molded body disclosed in Korean Patent No. 10-1229064 (Document 1).
In the invention disclosed in Document 1, a specimen, which is in the form of a sheet and made of an amorphous alloy, is manufactured by placing an alloy material on a support having a flat upper surface, heating and melting the alloy material by using a heater above the alloy material, removing the heater while moving the support upward, moving a chill member upward such that the molten metal is placed between a lower surface of the chill member and an upper surface of the support, and cooling the molten metal by transferring heat to the chill member and the support.
According to the disclosure in Document 1, a small sheet made of an amorphous alloy at a specimen level is manufactured by using the apparatus and the method, but there are problems in that the manufactured sheet has a very irregular surface state such that the manufactured sheet needs to be machined to make a specimen by cutting and removing a significantly large part of a surface of a casted product, quality of the casted specimen is very irregular because the casting is not performed in a sealed mold, and as a result, it is impossible to obtain a shape required as a final product.
Further, according to the method and the apparatus according to the invention disclosed in Document 1, a driving device, which operates the support, the heater, and the chill member, is additionally required in comparison with the casting mold of the type illustrated in FIG. 1, and as a result, there are problems in that a configuration of the apparatus and a control method are very complicated and a large amount of costs is required.